Inventory log: Add or Subtract values from another sheet

I need help, attached is a simplified copy of what i'm trying to do. I already have a spreadsheet that does all this but based on how many different classes and subclasses I have it became a workbook with multiple sheets. I'm trying to simplify it and break it down to 2 sheets, problem is Im sure i'll need to use VBA which im not good at. Any helpers?

I have a starting available value, I need to add or subtract from that value based selecting "consumed" or "purchased" on a different sheet. I need the formula to continue going consistently as I continue adding to my log.

For example:

If I have 100 of [Class] Soap, [Subclass] A
I "Consumed" 50
My new "Available Value" = 50

on the next day I add to my log that I "purchased" 50 of [Class] Soap, [Subclass] A
My new "Available Value" = 100

There are multiple [Classes] and [Subclasses]

Let me know if you need more clarification! Thanks in advanced!